1. Field of the Invention
This invention relates to a holographic laser beam scanning apparatus, and more particularly to a holographic laser beam scanning apparatus which permits astigmatism to be corrected by conforming a wave front of a collimated laser beam to a wave front of a laser beam used in making a hologram of a hologram disc.
2. Description of the Prior Art
Holographic laser beam scanning apparatuses are generally used in a variety of electronic products, for example, a laser beam printer, a bar code reader, a laser display, etc., which utilize a laser beam, and in recent years the range of use of such apparatuses has been progressively enlarged.
The known holographic laser beam scanning apparatus is of the type shown in FIG. 3 of the accompanying drawings, in which a laser beam B outputted from a laser generator 1 passes through a collimator 2 to be converted into a parallel beam, which is then reflected at an angle of 90 degrees by means of a reflecting mirror 3 to a hologram disc 4. The hologram disc 4 is rotatingly driven by a motor 6 to scan the reflected beam L incident on the hologram 5 of the disc, as a diffracted beam W having a given angle of diffraction, thereby forming a focus at a point P.
This prior apparatus however has a drawback in that since a wave front of the laser beam generated by the laser generator 1 and incident on the hologram disc 4 does not exactly coincide with a wave front of the laser beam used in making the hologram constituting the hologram disc, the beam diffracted through the hologram includes astigmatism created due to discordance of the wave fronts, so that a focus having a desired size may not be obtained. Therefore, when the beam is used in apparatuses needing precise scanning, precision and reliability of operation of the apparatus may be decreased.
Such a problem is caused by the following fact. In order to make the hologram, parallel light L.sub.1 and a focusing spherical wave W.sub.1 having a focus at a given point P.sub.1 are directed to the hologram 5 at a given angle .alpha..sub.1, as shown in FIG. 4. When the parallel beam is incident on the thus formed hologram 5 in the holographic laser beam scanning apparatus shown in FIG. 3, the diffracted beam passed through the hologram 5 has astigmatism resulting from discordance of beam L generated by the laser generator 1 and incident on the hologram 5 through the collimator 2 and the reflecting mirror 3.